The referenced Publication "Siemens Components 24", Issue 1, 1986, pp. 9-13,30, describes an active harmonic filter for use in network power rectifier circuits. Active harmonic filters permit connection to a power network so that the current accepted from the power network will be essentially sinusoidal, with a power factor of almost or approximately unity. The d.c. output voltage should be capable of being regulated.
The harmonic filter, as described, basically includes a network rectifier with an input radio interference suppression filter. The rectifier circuit is connected downstream of the interference suppression filter. It operates as a step-up circuit with low input capacitance.
Basically, the active harmonic filter includes an inductance connected between the output of a rectifier, for example a typical bridge rectifier, and d.c. output terminals of the power supply. A power storage capacitor is connected across the output terminals, in well-known fashion, and a diode is interposed between the connection from the inductance, for example a choke, to the respective d.c. output terminal. A controlled electronic switch, typically a transistor, is connected between the diode--inductance connection junction--and the other d.c. output terminal of the power supply. A control circuit is connected to the transistor which so controls conduction of the transistor that the input current taken by the overall circuit from the a.c. power network is as sinusoidal as possible, while providing at the output an output voltage and current which is regulated, independently of voltage variations of input voltage and essentially ripple-free. To do so, the inductance or choke must not have any gaps in current flowing therethrough, since, otherwise, the current accepted from the power network would no longer be sinusoidal. This requires switching of the conduction state of the cross-connected transistor only when the choke or inductance is discharged, that is, no longer stores any electromagnetic energy.
The charge state of the inductance can be sensed by winding the inductance with a secondary winding or sensing winding. If the cross-connected transistor is connected when the choke is discharged, and with constant turn-on-time, a network current will result which is proportional to network voltage, that is, under normal loading condition will be sinusoidal. The output voltage, or output current or power, respectively, can be controlled by controlling the connection period or duration of the transistor, and thus the electromagnetic storage time or charge time for the inductance. The off-time duration of the transistor is determined by the time to permit the inductance to discharge.
The on-time t.sub.1 of the transistor, in the circuit described in the referenced publication, is controlled by a comparatively complex circuit which receives information regarding the instantaneous input voltage U.sub.E to the control circuit, derived from the output of the rectifier, and the instantaneous output voltage U.sub.o from the power supply, as well as a signal representative of the instantaneous current flowing through the transistor. The secondary winding of the choke provides a signal which is indicative of complete discharge of the electromagnetic energy charged in the choke. The circuit includes control amplifiers, comparators comparing the instantaneous output voltage with a command output voltage, a multiplier which receives the output difference value of a first control amplifier and the instantaneous input voltage, a second control amplifier which compares the actual current with a command current through the transistor, and which provides an output signal to control a driver stage. The driver stage includes an amplifier, a bistable flipflop circuit and connecting elements. While the control circuit works well, it requires a substantial number of components and is comparatieely complex. It has been found that the exact instant of time of discharge of the inductance can be sensed by the secondary winding only with great difficulty, so that controlling the cross-connected transistor to conduction is not precisely determined. If the cross-connected transistor is controlled to conduction too early, a short-circuit current from the output capacitor may flow through the transistor due to the reverse recovery period of the serially connected diode. This current may reach a dangerous level, damaging or even destroying the transistor and/or the diode, while, further, reducing the efficiency of the step-up circuit. Additionally if the OFF time of the transistor is longer than the discharge period of the choke, to reliably prevent any short-circuit currents, undesired oscillations may occur based on the leakage inductance of the choke and parasitic capacities of the transistor which, again, reduce the efficiency of the circuit network. Further, undesired current gaps may occur which distort the pure sine wave of current being supplied to the power supply.